The invention refers to the field of technical optics and is directed to an optical color-splitter arrangement in an optoelectronic color scanner for recognizing or, respectively, for separating colors scanned in a color area.
Such an optoelectronic color scanner is essentially composed of the optical color-splitter arrangement for the individual color channels for resolving the light reflected by or allowed to pass by the scanned color area into at least two color components of different spectral ranges, usually into three spectral color components "red", "green" and "blue", and of optoelectronic transducers with which the spectral color components are converted into electrical color signals.
Dichroitic mirrors are predominantly employed for spectral resolution of the light, these having the properties of reflecting, i.e. blocking, light of a limited spectral range and allowing light of the remaining spectral range to pass. In order to achieve a good color selectivity of the color-splitter arrangement, the pass ranges and blocking ranges of the filter curves of the dichroitic mirrors must be optimally well-tuned to the spectral ranges of the light to be separated from one another.
The spectral filter curves of commercially obtainable dichroitic mirrors, however, have manufacture-caused tolerances of the filter edges between pass ranges and blocking ranges.
In order to create precisely defined spectral ranges for the color-splitter arrangement and in order to enhance the color selectivity by constricting the spectral ranges, correction means are allocated to every color channel of the color-splitter arrangement, the edges of the filtered curves of the dichroitic mirrors being capable of being corrected with these correction means. Glasses having colored additives, and which are referred to as colored glass filters, or glasses that absorb short-wave light, and which are referred to as stop glasses, are employed as correction means.
A correction or respectively, a balancing of the spectral ranges of the color-splitter arrangement under measurement control is not only required in the manufacture of the color-splitter arrangement but is also required given every replacement of a dichroitic mirror or of one of the optoelectronic transducers of the color scanner since the spectral sensitivity distribution of the entire color scanner can change when replacing an optoelectronic transducer.
Practice has shown that the spectral balancing of a color-splitter arrangement is time-consuming and also cost-intensive since a considerable expense for measurement is required and a great number of colored glass filters or stop glasses having different filter curves must be kept on hand.
A further disadvantage is comprised therein that high light losses due to absorption in the glasses can arise given the employment of colored glass filters and stop glasses as correction means. For compensating these light losses, the amplification of the color signals must then be increased, as a result whereof the noise part in the color signals rises and the color selectivity of the color scanner decreases.